I'm pleased to know that you are planning to implement the participation/ engagement rubric. I'd be very interested in hearing about your perspectives.
For this rubric, we used the 5-point scale without an explanatory text for each of the ratings. However, we explained that 5 was represented by all or nearly all of the descriptive criteria provided.
We presented this rubric as part of the introductory class when we were delving into the syllabus and the course learning outcomes. Therefore, learners could assemble their evidence throughout the semester, though many did that just prior to handing in the assessment, as is the case for most of us in similar situations. We discussed the nature of participation and why that is important for their learning. For some classes, I asked the learners to review and enhance the criteria.
The evidence comprised a self-assessment of their learning. We asked them to describe events, assignments, discussions or applications of the ideas that helped them determine that they were demonstrating that professional quality. Based on that evidence, we asked the learners to self-assess on the rating scale.
Prior to reading their self-assessments of their participation/ engagement, I recorded an assessment for each of the learners based on my notes and experiences of their involvement in the class. Then I'd read their self-assessments.
About 80% of the time, their and my assessments were in agreement. Some students did not give themselves full credit for the quality of their engagement, and then they would receive my higher ratings, along with my explanation. Occasionally, students gave themselves more credit for the quality of their participation than I felt was due. In that case, I set up a private meeting with the student so that we could discuss the situation.
This process was both rewarding and time-consuming, however it does emphasize the intent of learners taking responsibility for their own learning.